In an automobile suspension system, tubular vibration isolating bushings have been used for the purposes of vibration damping and shock absorbing at connecting parts of the vehicle body and the suspension, namely, each connecting part between a support member such as a control arm (lower arm, etc.) carrying wheels, and a body side member such as body frame and the like. Besides, the tubular vibration isolating bushings may also be used for an engine mount for supporting an engine to isolate vibration.
FIG. 14 shows a conventional vibration isolating bushing 100. The bushing 100 comprises an inner cylinder 101 and an outer cylinder 102, both made of a metal and disposed concentrically, and a rubber elastomer 103 interposed between both cylinders. As for the vibration isolating bushing 100, in the state of use, an axial member 104 is inserted into the inner cylinder 101, and the vibration isolating bushing 100 is fastened and secured to support members 105 such as brackets that hold both ends of inner cylinder 101 in between. The outer cylinder 102 is press-fitted and secured against an attachment hole 107 of other support members 106.
Thus, the inner cylinder 101 of the vibration isolating bushing 100 is held at both ends between the support members 105. Accordingly, if the areas of the terminal edges 108 of the inner cylinder 101 are small, their surface pressures receiving an axial force due to fastening are high, the support members 105 such as brackets, which are press-formed out of plate materials, are depressed, giving rise to problems such as looseness of bolts, etc.
Therefore, in order to inhibit surface pressures against the axial force due to fastening at the terminal edges less than the predetermined level, a technique is taken by entirely thickening the thickness of the inner cylinder 101 assuming a straight form and also by enlarging the areas of the terminal edges 108 constituting a contact surface with the support member 105. However, in case where a thick inner cylinder 101 is used, although the areas of the terminal edges 108 of the inner cylinder 101 receiving the axial force grow larger, there is a drawback in that weight is increased greatly.
As shown in FIG. 15, there is also a technique of enlarging the areas of the terminal edges 108 constituting a contact surface with the support member 105 by making use of an oddly shaped inner cylinder 101, both extremities 109 of which are formed thicker than an inward portion. However, in this case, the oddly shaped inner cylinder 101 is preformed by one of the working methods such as forging, etc., accordingly leading to higher cost. Further, when the rubber elastomer 103 is vulcanization formed, also in connection with die release after forming, a free length cannot be secured sufficiently at the terminal edges 110 of the rubber elastomer 103, so that there is a problem that durability grows worse. That is, when the extremities 109 of the inner cylinder 101 are previously formed to be upset, an inward portion 111 of smaller diameter than that at the upset extremities 109 assumes an undercut shape against die release in the axial direction, with the result that the terminal edges 110 of the rubber elastomer 103 cannot be set to the position indicated by a two-point chain line 112. Therefore, the areas of the rubber elastomer 103 at the terminal edges 110 become smaller, a free length from elastic deformation cannot be secured sufficiently.
As shown in FIG. 16, it is disclosed in an official gazette JP-A-5-200438 that the extremities 109 of the inner cylinder 101 are formed to be upset by subjecting to cold plastic working with the aid of an upsetting jig 120, after vulcanization forming of the rubber elastomer 103, to enlarge the terminal edges 108. In the official gazette, the upsetting jig 120 having a protrusion 121 at the center on the tip surface is employed. By pressing the upsetting jig 120 against the terminal edges 108 of the inner cylinder while turning the upsetting jig 120 in a conical orbit centering an axis 122 of the inner cylinder 101, the terminal edges 108 are enlarged as shown in FIG. 17.
According to this method, the areas of the terminal edges 108 of the inner cylinder 101 can be made larger without using a thick inner cylinder, and the areas of the terminal edges 110 of the rubber elastomer 103 can also be enlarged to secure sufficiently a free length from elastic deformation. However, according to the upsetting method disclosed in the official gazette, since not only outside diameter D0 but also inside diameter d0 of the inner cylinder 101 at the extremities 109 are upset as shown in FIG. 17, the areas of the inner cylinder at the terminal edges 108 become narrower by the upset area of the inside diameter d0. Accordingly, it is difficult to secure sufficient area to reduce surface pressure for the axial force described above.
As shown in FIG. 14, when the axial member 104 having a bolt 130 at the tip is inserted into the inner cylinder 101 and fastened and secured to the support member 105 by a nut 131, it is preferably to diminish the inside diameter at the extremities of the inner cylinder 101 for positioning the bolt 130. However, since the inside diameter d0 is also upset according to the method disclosed in the official gazette, it is not possible to perform such positioning. Further, a problem is encountered that it is difficult to diminish the inside diameter of the inner cylinder at the extremities during forging process.